In sewage treatment plants and industrial wastewater treatment plants the most common method of providing mass transfer of oxygen to the process is through diffused aeration with flexible membrane diffusers.
Such diffusers are typically manufactured of a plastic housing and can have either an EPDM (Ethylene Propylene Diene Monomer) membrane or a silicone rubber membrane. These membranes are typically perforated with numerous small slits or holes or openings. The completed diffusers having the membrane are then attached to a piping system containing compressed air and installed, typically, at the bottom of a tank, with the compressed air providing the air flow as needed. The small slits or openings or perforations vibrate open and close continuously as air passes through them, creating fine bubbles. When the compressed air is shut off, the slits seal, and the membrane is pressed against the plastic holder, and functions like a check valve preventing water from entering the air piping system.
Conventional membranes made out of materials like EPDM work well in a large number of treatment plants, however, it has been discovered that they can clog-up or be attacked, such as, from most solvents, fats, oils, greases, acids, bases, to name a few.
U.S. Pat. No. 4,491,653 (Vincent C. McGinniss, et al.) the disclosure of which is incorporated herein by reference, discloses a method for fluorinating the surface of a polymeric solid for improving a surface property of such solid while substantially maintaining the bulk physical properties of said solid. This method comprises contacting the polymeric solid with dilute fluorine gas at a temperature not substantially above about room temperature and at a pressure not substantially above about 1 atmosphere for a time adequate to partially fluorinate said solid surface by forming stable fluorocarbon groups wherein the ratio of —CF.sub.2— groups to —CHF— groups is not substantially above about 1:1, the proportion of oxygen-providing groups pre-send during said fluorination being restricted to an amount whereby substantially no oxidation of said surface occurs.
U.S. Pat. No. 4,869,859 (Manfred Eschwey, et al.) the disclosure of which is incorporated herein by reference, discloses a process for blow molding and fluorination of a hollow thermoplastic article includes bringing a blank to rest against the inside wall of the blow mold as a result of introducing an inert blow gas. An inert gas/reaction mixture is then introduced in the hollow article for fluorinating the inside surface of the hollow article. After the inside surface of the hollow article has cooled off, the inert gas/reaction gas mixture is blown in below the melting point of the thermoplastic material.
U.S. Patent Publication No. 2005/0003204 (Thomas E. Frankel, et al.), the disclosure of which is incorporated herein by reference, discloses a multi-layer article includes a fluoropolymer layer and a substrate which are joined permanently through a multi-step cross-linking technique. Also, disclosed is a multi-layer membrane with a molded fluoro-elastomeric surface layer.
U.S. Patent Publication No. 2007/0001323 (Seoungil Kang), the disclosure of which is incorporated herein by reference, discloses a multi-layered membrane of disk-typed and tube-typed membrane air diffuser. The system is installed in the aeration tank of the sewage and waste water treatment system that is used to turn the air, which was supplied from the blower, into a fine bubble spray so that the oxygen in the air can be efficiently dissolved into the sewage and waste water. By doing it, this system analyzes and removes the biochemical oxygen demand and suspended solids biologically, and enables to be appropriately mixed so that the sedimentation within the aeration tank can be prevented. Also, disclosed is a multi-layer membrane with a coated fluoro-elastomeric surface layer.
U.S. Patent Publication No. 2007/0128394 (Thomas E. Frankel, et al.), the disclosure of which is incorporated herein by reference, discloses a multi-layer membrane which includes a fluorine containing layer and a substrate which are joined through spray coating and heat treatment in a multi-step technique. Also, disclosed is a multi-layer membrane with a coated fluoro-elastomeric surface layer.
Furthermore, most molded elastomeric membranes of any material have a rough surface when viewed under a microscope. This roughness of the surface allows foulants in the wastewater, such as, calcium carbonate, to deposit and collect on the surface. Such deposits can significantly impair the efficiency of the diffuser.
It has also been discovered that solvents that are used to apply the coating on these diffuser membranes also create issues relating to the use of these coatings in wastewater applications, such as, membrane coatings containing solvents have been known to dissolve over time where like solvents are present in the wastewater.
It has also been observed that membranes of the prior art do not protect against inner-slit fouling by foulant materials, such as, for example, calcium carbonate.
Even with these improvements, and known problems of the prior art, a need exists for an improved gas membrane diffuser.
Thus, a need exists for an improved diffused aeration system.
A need also exists for a diffuser membrane where the surface has been treated with at least one layer of fluorine.
The invention provides a diffuser membrane where the surface has been treated with at least one layer of fluorine.